Radio frequency (RF) transmitters, such as those included in mobile wireless telephone handsets (also referred to as cellular telephones) and other portable radio transceivers, generally include a power amplifier. The power amplifier is typically the final stage of the transmitter circuitry. In some types of transmitters, achieving linear power amplification is of great importance. However, various factors can hamper linear operation. For example, in a transmitter of the type generally included in some types of mobile wireless telephone handsets, where the power amplifier receives the output of an upconversion mixer, the relatively large signal that such a mixer typically outputs can drive the power amplifier into nonlinear operation. Increasing power amplifier current is one technique for promoting linear operation in such a transmitter, but it does not work well in all instances.
As illustrated in FIGS. 1-2, in a transmitter of the type generally included in some types of mobile wireless telephone handsets, the power amplifier 10 typically comprises several amplifier driver stages or sections 12, 14, 16, etc., at least one of which, such as amplifier driver stage 14, comprises a transconductance (Gm) amplifier that outputs a radio frequency (RF) current signal 18 (I_OUT) in response to an RF input voltage signal 20 (V_IN). The gain of power amplifier 10 can be controlled by controlling the bias voltage signal 22 (V_BIAS), which is provided via an RF choke 24. (Although not shown in FIGS. 1-2 for purposes of clarity, circuitry in the mobile wireless telephone handset generates bias voltage signal 22 in response to various operating conditions that require adjusting transmitter output power.) As illustrated in FIG. 2, the transconductance amplifier transistor 26 is typically a metal oxide semiconductor field-effect transistor (MOSFET) arranged in a circuit in a common-source configuration. The RF input voltage signal 20 is coupled to the gate of transistor 26 via a coupling capacitor 28. Current source circuitry that is coupled to transistor 26 is not shown for purposes of clarity but is indicated by the ellipsis (“ . . . ”) symbol. Such a MOSFET, when driven by a relatively large signal, produces a nonlinear current signal 18 as a result of transistor effects such as mobility degradation, velocity saturation, and nonlinearity of the input capacitance. It is known to design transconductance amplifiers to operate at increased current levels in an attempt to meet noise performance requirements and to some extent promote linear operation. However, increasing current alone generally cannot provide sufficient overdrive voltage at the gate-source junction to render a linear output current signal 18. A technique known as degeneration can be combined with the above-described increased current technique to further promote linearity, but degeneration hampers the use of bias voltage signal 22 as an amplifier gain control. Also, increasing current in a mobile wireless telephone handset power amplifier tends to more quickly drain the battery.
It would be desirable to promote transconductance amplifier linearity in a manner that does not consume excessive current, degrade amplifier noise performance, or sacrifice bias voltage gain controllability.